Make the slaw: In a small saucepan, combine the water, sugar, and vinegar and bring to a boil. Transfer the vinegar mixture to a bowl and cool. Add the carrot and daikon, mix well, and season with salt. Set aside to marinate for 30 minutes or store in the refrigerator up to overnight.
Meanwhile, make the seasoned pork: Heat the oil in small nonstick skillet over medium heat. Add the onion and cook, stirring, until soft. Add the pork, seasoning, garlic, and pepper and cook, stirring, until just cooked through, about 2 to 3 minutes. Remove the heat and set aside covered with foil to keep warm.
Make the sandwiches: Preheat oven to 400 degrees F. Slice the baguettes open lengthwise, and slather the insides with mayonnaise. Arrange the baguettes on a baking sheet and bake until hot and crusty about 5 minutes. Remove the baguettes from the oven and immediately fill each with some of the seasoned pork. In each sandwich, arrange 2 slices each of the pork roll and salami, 1 teaspoon soy sauce, 1/2 tablespoon cilantro, 1 slice cucumber, ground pepper, and chili oil, if using. Serve immediately with the slaw on the side.
*available in Asian markets
Recipe Courtesy of Anthony Nguyen, Saigon Banh Mi So # 1, NYC